Day 3 vs Day 5 Embryo Transfer in IVF
During an IVF cycle, one of the decisions that often surprises patients is timing: the embryo can be transferred into the uterus a few days after egg collection, or the laboratory can keep growing it for a couple more days first. These two options are commonly called a day 3 transfer and a day 5 transfer. Neither is automatically "better" for everyone. The right choice depends on how many embryos are developing, the patient's age and history, the laboratory's experience, and whether embryos will be frozen. This article explains what the terms mean, what current research shows, and what trade-offs are worth understanding.
What the Day Numbers Mean
In IVF laboratories, the day of egg collection and fertilisation is counted as day 0. An embryo is then watched as it divides.
By day 3 an embryo is at the cleavage stage. It is usually made up of roughly six to eight cells that look similar to each other. At this point the embryo is still relying largely on materials stored in the egg, and its own genes are only starting to take over.
By day 5 (sometimes day 6, and occasionally day 7) a healthy embryo can reach the blastocyst stage. A blastocyst has many more cells arranged into two distinct groups: the inner cell mass, which can go on to form the baby, and the trophectoderm, which can go on to form the placenta. A fluid-filled cavity develops in the middle. Not every embryo gets this far, which is a normal part of early human development rather than a sign that something was done wrong.
Why Extended Culture Is Used at All
The uterine lining is naturally receptive to an embryo at about the time a blastocyst would be present, so transferring at the blastocyst stage fits the body's timing more closely. In addition, the extra days allow the laboratory team to see which embryos keep developing, which can help when choosing which embryo to transfer or freeze. This is a practical argument rather than a guarantee: appearance on a given day is an imperfect guide to an embryo's potential, and no laboratory marker can reliably tell in advance which day-3 embryos will become blastocysts.
What the Research Shows
A 2026 Cochrane systematic review (36 randomised trials, 8,389 participants) compared blastocyst-stage with cleavage-stage transfer. Its findings, in plain terms:
- Live birth per fresh transfer was probably higher with blastocyst transfer (odds ratio 1.39; moderate certainty), and clinical pregnancy per fresh transfer may also be higher (low certainty).
- Cumulative live birth within 12 months, which counts fresh and frozen transfers from the same egg collection, may be higher with blastocyst transfer (odds ratio 1.24), but the certainty is low and the review notes it remains unclear whether the difference holds up.
- There was little or no difference in miscarriage per fresh transfer.
- Preterm birth (before 37 weeks) was probably higher after blastocyst transfer on a cumulative basis (odds ratio 1.72; moderate certainty).
- Blastocyst cycles may leave fewer spare embryos to freeze, and they may be more likely to end without any embryo being transferred (low certainty for both).
The review's authors emphasise that these findings come mainly from women with a good prognosis and should not be generalised to women with a poorer prognosis. An odds ratio also does not translate directly into a percentage difference for any individual; it describes how the odds shifted across groups of patients in trials.
The American Society for Reproductive Medicine (ASRM) reached a broadly similar position in its committee opinion on blastocyst culture and transfer. It supports blastocyst transfer in good-prognosis patients, notes that the apparent live-birth advantage became weaker when only the best-designed trials were considered, and states that cumulative delivery rate per egg collection is the most meaningful measure of overall success. It also highlights that much of the neonatal benefit once attributed to blastocyst transfer reflects greater use of single embryo transfer, rather than the culture stage itself.
Possible Advantages of a Day 5 Transfer
Better Timing with the Uterus
The embryo arrives at a stage that matches the lining's natural receptivity.
More Information Before Choosing
Embryos that stop developing before the blastocyst stage can be identified, so the transferred embryo has already shown continued development.
Support for Single Embryo Transfer
When a good-quality blastocyst is available, transferring one embryo is more reasonable, which lowers the chance of twins.
Compatibility with Embryo Biopsy
If genetic testing of embryos is planned for a recognised indication, a blastocyst is the stage at which a small number of cells are usually sampled.
Possible Disadvantages and Uncertainties
No Embryo May Reach the Blastocyst Stage
This is the main concern. In a large ASRM review of unselected patients, cancelled transfers were more frequent with blastocyst culture (about 8.9% versus 2.8%). Blastocyst formation varies widely between individuals, and the day-3 appearance does not reliably predict it.
Fewer Embryos to Freeze
Embryos that might have been transferred or stored at day 3 may not survive extended culture. Whether those embryos would ever have produced a baby is not always clear, but the practical effect is a smaller number of embryos to work with.
Preterm Birth
The Cochrane review found higher cumulative preterm birth after blastocyst transfer. ASRM describes the absolute risk difference in the studies it reviewed as small and notes the cause is unclear; it may reflect patient selection, culture conditions or other factors.
Identical Twinning
ASRM reports that most, though not all, studies show a small increase in monozygotic (identical) twinning after blastocyst transfer. Patients are advised to be counselled that the absolute increase is small.
Possible Effects of Extended Culture
Some studies have suggested that prolonged culture may influence gene expression and epigenetic marks, whereas others have been reassuring. The mechanisms are not established, and this is an area where more research is needed.
When a Day 3 Transfer May Still Make Sense
A day 3 transfer may be considered when only a few embryos are available and the team is concerned that none would survive extended culture, when previous cycles ended without a blastocyst, or when the laboratory's experience and conditions make an earlier transfer a more cautious choice. A small retrospective single-programme review reported that, for patients with no eight-cell embryos on day 3, only day 3 transfers resulted in pregnancy, and that benefit of day 5 transfer was unclear when only one or two good embryos were present; findings like this are hypothesis-generating rather than definitive, but they illustrate why a one-size-fits-all rule is not used.
The decision also depends on whether embryos will be transferred fresh or frozen. If the plan is to freeze embryos and transfer later, the stage at which they are frozen is a separate laboratory decision.
How Many Embryos Should Be Transferred?
The stage of the embryo and the number transferred are linked. Multiple pregnancy (twins or more) carries higher risks for both the pregnant person and the babies, including preterm birth, so most current guidance aims for a healthy single baby.
The UK's National Institute for Health and Care Excellence (NICE) guideline NG257 advises single embryo transfer where a top-quality blastocyst is available, and says no more than two embryos should be transferred in any one cycle. Its recommendations for the number of embryos vary with age and the number of previous full cycles.
ASRM and the Society for Assisted Reproductive Technology recommend transferring one embryo when the embryo is known to have a normal chromosome count, and strongly encourage single embryo transfer in patients under 35. Their upper limits are generally lower for blastocysts than for cleavage-stage embryos at the same age and prognosis, because blastocysts tend to have higher implantation potential.
Where a pregnancy could be especially risky because of a medical condition, a single embryo is advised. With gestational carriers, single embryo transfer is strongly recommended.
The shift towards single embryo transfer is visible in national data. In the United Kingdom, the Human Fertilisation and Embryology Authority reported that 84% of IVF transfers in 2024 were single embryo transfers and that the multiple birth rate had fallen from 14.4% in 2014 to 3.2% in 2024. Frozen embryo transfers made up 48% of IVF cycles in 2024, up from 24% a decade earlier. These figures describe national trends and cannot be used to predict an individual outcome.
Embryo Grading and Genetic Testing: What the Day Does Not Tell You
Whichever day is chosen, the appearance of an embryo is not the same as its chromosome status. Many embryos with a good appearance have an abnormal chromosome count, particularly as age increases. Testing of embryos for chromosome numbers (PGT-A) is a separate topic. ASRM's 2024 committee opinion concludes that the value of PGT-A as a routine screening test for all IVF patients has not been demonstrated, and NICE advises against offering PGT-A to improve live birth rates. Choosing day 5 so that testing can be done is therefore not a reason by itself to extend culture.
Questions That Help When Discussing Timing
- How many embryos are developing, and how do they look at this point?
- What proportion of embryos in my situation typically reach the blastocyst stage in this laboratory?
- If no embryo reaches the blastocyst stage, what are the options?
- Are any embryos likely to be frozen, and at which stage?
- How many embryos would be transferred, and what are the risks of twins in my situation?
Key Points
Day 3 and day 5 transfers are both established parts of IVF care. For patients with several good-quality embryos, large reviews suggest blastocyst transfer probably improves the chance of live birth per fresh transfer, although benefits per egg collection are less certain, and there may be a small increase in preterm birth. For patients with few embryos, extended culture carries a real possibility that no embryo is available to transfer or freeze, and the evidence in poorer-prognosis groups is limited. The most useful approach is an individual plan made by the treating team, based on the number and quality of embryos, age, previous cycles and the laboratory's own results, with the shared aim of a healthy single pregnancy.
References
- Glujovsky D, Cornelisse S, Blake D, Quinteiro Retamar AM, Alvarez Sedo CR, Ciapponi A. Blastocyst-stage versus cleavage-stage embryo transfer in assisted reproductive technology (update). Cochrane Database of Systematic Reviews, Issue 1, 2026, CD002118.pub7. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD002118.pub7
- Practice Committees of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology. Blastocyst culture and transfer in clinically assisted reproduction: a committee opinion. Fertility and Sterility, 2018;110:1246-1252. https://www.asrm.org/practice-guidance/practice-committee-documents/blastocyst-culture-and-transfer-in-clinically-assisted-reproduction-a-committee-opinion-20182/
- Practice Committees of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology. Guidance on the limits to the number of embryos to transfer: a committee opinion. American Society for Reproductive Medicine, 2021. https://www.asrm.org/practice-guidance/practice-committee-documents/guidance-on-the-limits-to-the-number-of-embryos-to-transfer-a---committee-opinion-2021/
- National Institute for Health and Care Excellence. Fertility problems: assessment and treatment (NG257), procedures used during in vitro fertilisation (IVF). NICE, 2026. https://www.nice.org.uk/guidance/NG257/chapter/procedures-used-during-in-vitro-fertilisation-ivf
- Human Fertilisation and Embryology Authority. Fertility treatment 2024: trends and figures. HFEA, 2026. https://www.hfea.gov.uk/about-us/publications/research-and-data/fertility-treatment-2024-trends-and-figures
- Practice Committee of the American Society for Reproductive Medicine. The use of preimplantation genetic testing for aneuploidy (PGT-A): a committee opinion. American Society for Reproductive Medicine, 2024. https://www.asrm.org/practice-guidance/practice-committee-documents/the-use-of-preimplantation-genetic-testing-for-aneuploidy-a-committee-opinion-2024